The present invention relates to nuclear fuel assemblies, and more particularly, to nuclear fuel assembly spacer grids.
In a light-water nuclear power reactor, a multiplicity of elongated nuclear fuel assemblies are supported in a close, side-by-side relationship. Heat generated in the fuel assembly is transferred to pressurized water, which flows longitudinally through the assemblies. Each assembly typically has a plurality of elongated support tubes to which are rigidly attached a plurality of longitudinally spaced apart, transversely oriented grids. The grids are typically formed by orthogonally oriented, metal strips interlaced in egg-grate fashion, so as to define an array of channels or cells. Fuel rods traverse the assembly through successive grids. The walls of each cell include fuel rods support structure, for maintaining the fuel rods in a predetermined, uniform spacing from each other, thereby optimizing the neutron physics and heat transfer within the assembly and the core as a whole.
Despite the ideal objective of minimizing the peak to average power ratio throughout the core, and the heat transferred to the coolant along the length of the "hottest" fuel rod, the geometry and heterogeneous nature of the core necessarily produces non-uniformities in flow and power generation. Such non-uniformities are particularly evident within each fuel assembly, immediately surrounding the guide or support tube, because such tube interrupts the regular array of fuel rods and the coolant flow cross-sectional area associated with the average rod. Moreover, the coolant flow surrounding the guide tube tends to be higher than the average coolant flow through the assembly, whereas the power generated in the channel or cell containing the guide tube, is zero, due to the absence of fuel therein.
Attempts have been made in the past to affect the coolant flow distribution surrounding the guide tube. In U.S. Pat. No. 3,787,286, issued Jan. 22, 1974 to Anthony, metal strips containing pre-punched tab projections are attached to at least some of the guide tubes, and include projections into the tube flow channel within the grid. U.S. Pat. No. 4,775,510 issued Oct. 4, 1988 to Bryan, describes a hollow flow deflector in the form of a sleeve which is attached to the guide tube at locations between the grids. Both flow deflecting structures described in these patents are attached directly to the guide tubes, and thus require additional fabrication steps during manufacture of the grids.